1. Technical Field
The present invention relates generally to a vehicle power transmission device equipped with a power split device made up of a plurality of power split rotors which interlock with each other to split output power or torque between a power source and a driven wheel and a speed variator coupled to the power split device.
2. Background Art
Japanese Patent First Publication No. 2006-308039 teaches a power transmission device made of a combination of two planetary gear sets and a continuously variable transmission (CVT). The power transmission device is equipped with a low-speed clutch and a high-speed clutch for use in switching between mechanical coupling modes between the planetary gear sets. When the power transmission device is required to run at a low speed, the low-speed clutch is engaged, while the high-speed clutch is disengaged, thereby creating circulation of power in the power transmission device to achieve the so-called geared neutral which places the speed of an output shaft of the power transmission device at zero (0) while keeping an input shaft thereof rotating. The power transmission device also works to regulate a gear ratio (also called a CVT ratio) of the continuously variable transmission to change the rotation of the output shaft from a normal direction to a reverse direction and vice verse across the geared neutral.
Usually, when an automotive vehicle runs at a low speed, for example, it starts on an uphill, the vehicle needs a great deal of torque, thus causing the torque exerted on the continuously variable transmission to be increased. The continuously variable transmission is, therefore, required to have an increased degree of durability. We have found that the greater a change in total gear ratio in a power transmission line between a power source and a driven wheel of the vehicle which arises from changing of the gear ratio of the continuously variable transmission, the greater the degree of torque acting on the continuously variable transmission. It is, therefore, difficult to achieve a good balance between establishment of a desired total gear ratio in the power transmission line and the durability of the continuously variable transmission.